Possible to fine-tune/offset frequency of radio (nrf51822) in code?

RE: Possible to fine-tune/offset frequency of radio (nrf51822) in code?

davman — Tue, 14 Oct 2014 23:16:22 GMT

Thanks for input Håkon, modules with dipole trace antennas are honestly only suitable for bigger products as any sensor / wearable and such will be too small that getting enough clearance around an antenna is impossible.

Back to my tuning question. Technically I guess this is actually doable if the SD could allow for setting relative offsets in the RF part, like calibration values that just compensates back a bit in software for modules making physical tuning impossible.

RE: Possible to fine-tune/offset frequency of radio (nrf51822) in code?

Håkon Alseth — Tue, 23 Sep 2014 06:12:07 GMT

You cannot change the output impedance of the radio. In order to tune this to your application you'll need to adjust the antenna itself. If you do not have any tuning components, and cutting the antenna is not an option, there is unfortunately not much you can do. Modules are made to be generic, and the antenna is one of the components that will suffer due to the different layout/setup. -2.8 dB is not bad (not really good either), especially if it's measured radiated.

RE: Possible to fine-tune/offset frequency of radio (nrf51822) in code?

davman — Mon, 22 Sep 2014 10:13:26 GMT

Hi Håkon,

Thanks for reply. The situation in our case is that the module & trace antenna we use doesn't allow any type of tuning (no external Pi-netw).. and hand cutting pcb-traces one-by-one in MP wouldn't make sense :-)

So, this is to me the problem with many modules, is the fact they are tunes with no plastic in close proximity. And in most use-cases (as in ours) it's hard to fit a module with more than ~1-2mm clearance to plastic.

What we see is that our antenna efficiency is around -2.8 dB at 2402, and worse higher up -3.52 dB at 2480 MHz.

My wish was if we in code could slightly shift, or rather compensate the base frequency to get the resonant back where it should be.

best david

RE: Possible to fine-tune/offset frequency of radio (nrf51822) in code?

Håkon Alseth — Thu, 18 Sep 2014 10:59:47 GMT

Hi,

Just to clarify: If the antenna is not 50 ohm in the 2.4GHz domain, it does not mean that you will get an output on a different frequency. It just means that the impedance match is off, and the performance (the radiated effect) in the 2.4GHz domain will be reduced because the resonance frequency of the antenna is not 2.4G.

If the module is pre-qualified, tuning the antenna will not require a re-qualification.

When you mention a shift of 150 MHz, what is the impedance of the antenna in 2.4/2.44/2.48 GHz? Note that the antenna shall always be tuned in it's application, meaning that if it's a heart rate monitor, it should be located inside the final housing and put up against your body when doing antenna related tuning.

We have a white paper on antenna tuning that may be of interest: www.nordicsemi.com/.../89766141

Cheers, Håkon

RE: Possible to fine-tune/offset frequency of radio (nrf51822) in code?

davman — Thu, 18 Sep 2014 07:54:22 GMT

Hi John, I would say the other way around, if a BLE module has been pre-certified and after assembly the radio is skewed slightly, to me it feels more reasonable to tune it back.

RE: Possible to fine-tune/offset frequency of radio (nrf51822) in code?

John — Wed, 17 Sep 2014 21:13:58 GMT

I have no idea if it is possible, although I've not seen anything in Nordic's documentation that would give you the ability to do that. That said, I suspect you might have problems with compliance bodies as the radio is no longer behaving as it was tested for intentional emitter compliance.